1. Field of the Invention
The invention pertains to sonar receive arrays. More particularly, the invention pertains to a process for calibrating the amplitude, position and phase angle of signals received from an array of underwater hydrophones with respect to one another. The array is linear and the hydrophones are distributed along the length thereof. Still more particularly, the invention pertains to the in situ measurement and calibration of various parameters which must be determined in order to calibrate and improve the performance of a towed array of receive hydrophones while the array is being towed through an underwater environment. The system operates by monitoring the responses of discrete hydrophones to generated acoustic test signals from a towed, stationary projector source.
2. Description of the Related Art
The gathering of data in an acoustic environment by means of an array of electrically interconnected hydrophones is important for commercial and military purposes. It is well known in the field of sonar systems to pull a submersible towed body behind a water craft. The towed body has one or more acoustic projectors associated with it. The towed body in turn draws behind it one or more flexible towed arrays of waterborne, signal receiving, hydrophone sensors. Such sensors are useful for detecting the position of underwater sonar target contacts.
Towed arrays of hydrophones are well known in the art. Such are long, continuous, neutrally buoyant hose lines, often several hundred meters long, which periodically contain numerous hydrophone receivers. Typically, these elongate arrays have a number of hydrophone elements in modules, along with their associated electronics and control leads, contained within the hose-shaped structure that extends through the water. In order to gather data in an ocean environment, one end of a linear hydrophone array is coupled to a navigating vessel, via the intermediate towed body. The individual hydrophones cooperate or interact to provide a beam, which may be steered to determine the bearing from the array of various acoustic targets. The hydrophones are connected to suitable electronics on the towing vessel which indicates the bearing of an identified underwater target. Exemplary towed arrays are described in U.S. Pat. Nos. 4,554,650 and 5,412,621. In locating submerged bodies, sonar is either used passively, through reception of signals sent through water, or actively wherein a pulse of sound is sent into the water by a sonar projector. The sound is then reflected back from the target and detected by the hydrophone arrays as an echo. This echo is received as mechanical energy by an acoustic transducer, converted into electrical energy, and read by a computer on the vessel to pinpoint the location of objects within the water. As shown in U.S. Pat. No. 5,844,860, elongated, hose-like towed arrays of hydrophones attached to the rear end of towed bodies are commonly used for the acoustic sensing of moving objects within the ocean.
Over a period of time, the detection capabilities of individual hydrophones in towed array may unpredictably vary. Consequently, to accurately interpret information provided by the array, calibration must be performed. In the prior art, various techniques are available for calibrating sections of a hydrophone array. In one approach to calibrating an acoustic array, the responses of individual hydrophones may be monitored, hydrophone by hydrophone, to determine their respective sensitivities to a test signal. However, due to the very large number of discrete hydrophones in an array, which may have a very long length, such calibration technique is extremely tedious. In addition, such calibration would have to be performed at a shore installation, so that changes in hydrophone characteristics occurring after deployment of the array at sea would not be possible. In another calibration approach, the sensitivity of individual hydrophones of an array are indirectly determined, while the array is deployed, by monitoring an acoustic source which is not located at or focused on any individual hydrophone. In this approach, multipath effects interfere with sensitivity measurements. In order to employ a towed array to determine the bearing of an acoustic source, it is necessary to know the positions of the respective hydrophones of the array in relation to one another. For such calibration, a calibration or test signal of selected frequency is projected to the array so that it is received simultaneously by every hydrophone of the array section. In cases where multiple hydrophones input an acoustic channel, the gross or collective response of the hydrophones to test signal includes the individual responses of defective hydrophones.
It is common practice in the art to assume that all of the hydrophones lie on the same line, at known distances along the array. However, such assumption is often invalid, such as when the towing vessel is unable to maintain a straight track through the ocean, or when a portion of the array sinks to a lower depth than another portion thereof. If the above assumption becomes inaccurate, the beamforming capability suffers and target bearing detection by means of the array may become unacceptably inaccurate. It would be desirable to directly calibrate individual hydrophones of a towed array during array deployment. U.S. Pat. No. 4,205,394, discloses one transportable calibration technique. U.S. Pat. No. 4,290,123 discloses an apparatus for determining the value of parameters of an interconnected array of acoustic sensor elements which includes a cart for successively bringing a projector into a closely spaced relationship with each of the sensor elements included in a length of the array. U.S. Pat. No. 4,267,585 discloses a device for determining the configuration of elongated, underwater arrays as they are drawn through the water, which comprises a collar adapted to fit around a portion of the array having sufficient hydrodynamic drag to remain substantially stationary in the water relative to the linear movement of the array as it is drawn through the collar. The use of such additional structures is disadvantageous.
Accordingly the invention provides a system which permits the inexpensive, expeditious and accurate determination of the characteristic underwater configuration of a towed acoustic array and which permits calibration and performance tests on each hydrophone in such underwater towed arrays.
This invention provides an improved system by which the detection capabilities of respective hydrophones lying along a towed array may be calibrated while the array is deployed in acoustic data gathering operations in deep water by calibrating each of the amplitude, position and phase angle of data received from the hydrophones relative to one another. The system uses an already in place projector of an active sonar system and hence does not require additional equipment.